Conventionally, in a manufacturing process for a semiconductor device, a hole such as a contact hole is formed in a silicon oxide film by plasma etching. When, for example, a resist is used as a mask for the plasma etching of the silicon oxide film, it is well known that a gaseous mixture of C4F6/O2/Ar is employed as an etching gas to improve the selectivity of the silicon oxide film against the resist (For example, see Japanese Patent Laid-open Application No. 2002-100607).
Furthermore, also known for the plasma etching of the silicon oxide film is a method for improving the in-surface uniformity of the plasma etching by changing a mixing ratio of Ar and He, which are used as dilution gases (see, for example, Japanese Patent Laid-open Application No. 2002-252213).
However, as semiconductor devices are getting miniaturized, the diameters of contact holes and the like are also required to be reduced to, for example, not greater than 70 nm. To meet such requirements, it is necessary to replace a conventionally employed KrF resist with an ArF resist. Since, however, plasma resistance of the ArF resist is weak, there has been made an attempt to plasma etch the silicon oxide film by using a hard mask made of, e.g., amorphous carbon. In this case, amorphous carbon is first plasma etched into a predetermined pattern by using the ArF resist as a mask, and plasma etching of the silicon oxide film is then conducted by using the amorphous carbon of the predetermined pattern as a mask.
When plasma etching the silicon oxide film by using the amorphous carbon as a mask, however, a sufficient selectivity of the silicon oxide film against the amorphous carbon (a ratio of an etching rate of the silicon oxide film to an etching rate of the amorphous carbon) cannot be obtained.
As described above, conventionally, when plasma etching the silicon oxide film by using the hard mask made of the amorphous carbon to form a fine pattern, there has been a problem that it is difficult to obtain a sufficient selectivity of silicon oxide film against the amorphous carbon (the ratio of the etching rate of the silicon oxide film to the etching rate of the amorphous carbon).